In order to move workpieces from one processing station to another roller conveyors commonly are used. Roller conveyors, such as shown in GB 788 892, consist of a frame upon which axially parallel rollers are rotatably mounted in side-by-side relation to each other. The workpieces run over the rollers, either directly on the rollers or by being attached to workpiece carriers.
At the output or downstream end of the roller conveyor, the workpieces should be in a waiting line with as few intervening gaps as possible. For the most part, the workpieces are not taken off at the same rate as they are supplied at the input end of the roller conveyor. Therefore, the workpieces must be moved from the input of the end roller conveyor to the end of the waiting line without being pulled off the conveyor. This inevitably leads to the situation where either the rollers slip under the workpiece carriers or workpieces, as is typical for roller conveyors, or where the rollers, which are in frictional contact with a workpiece or a workpiece carrier, become locked.
Due to cost reasons, it is not possible for each roller or each workpiece carrier to have its own drive. Rather, all of the rollers are simultaneously set in rotation by means of a transmission element in the form of a chain toothed belt, or a shaft.
Since slippage of the rollers under the workpieces or workpiece carriers is undesirable, stoppage of the roller is effected with the aid of frictional couplings. The frictional coupling for each roller acts like a gear between the given roller and the transmission element, which couples the roller to the drive motor. Disadvantages of frictional couplings include wear and tear of the components and the necessity of having to set the traction moment or the slippage moment by means of springs.
The problems with circular conveyors that transport pallets with the aid of endless-belt conveyor are similar. The pallets are led onto the conveyor offset from each other and are hauled by a transmission means in the form of a chain, a toothed belt, or the like. If the pallet stays in place, the pallet must not block the transmission means since other non-blocked pallets must continue to be moved. A given pallet is coupled by means of a gear to the transmission means, which begins to rotate as soon as the course of movement of the pallet is blocked. Frictional couplings also have been used for such circular conveyors.
The wear and tear on the frictional couplings is relatively high because the ratio between standby time of a roller or a workpiece carrier and the running time is relatively large. This means that the frictional couplings are in a slipping condition of operation most of the time.